1. Field of the Invention
In the prior art devices rolls having rotatable, perforated or cellular structure outer elements with an internal system for creating a sub-atmospheric pressure or vacuum within this rotatable outer element, are used for various purposes; such as, dewatering a moving, moist web of paper; applying greater traction for pulling a moving web of paper, metal or other materials; and picking up web or transferring sheet, an operation required in conjunction with certain printing presses. To produce stationary vacuum zones inside the roll, a suitable chamber connected to an external pump or exhauster must be provided, and in order to control the area in which vacuum is applied, as well as to assure optimum operating efficiency of the vacuum system, sealing arrangements must be provided to close off the line of contact between the inside of the rotating, perforated shell of the roll and the internally mounted stationary chamber. Presently a known practice is to mount rectangularly shaped pieces of non-metallic material having a low coefficient of friction and good wear qualities in a U-shaped member in such a manner that it can be radially forced outwardly against the internal surface of the rotating shell by springs or an inflatable air pressure tube. Of necessity, operating clearances must be provided between the non-metallic sealing element and the channel of the U-shaped holder in which it is disposed. In many applications of vacuum rolls certain parts thereof are exposed to contaminating environments, such as acids, adhesives or the like. Also, solid materials can become deposited in the clearances between the seal element and the U-shaped holder so that continuous free movement of the seal element becomes very difficult to maintain as the seal element is forced past the solid materials that are accumulated. As a result, the seal will become locked in a depressed position corresponding to the least internal radius of the rotating shell, and if there is any eccentricity in the bore of the shell, a clearance will occur when the shell is rotated to any other position and leakage past the seal will occur. In some applications of vacuum rolls, this can cause a fluctuation of the desired internal subatmospheric pressure in the roll with consequential adverse effect on the required function of the roll. Further, continued contact of the sealing element with the roll surface prevents proper cleaning thereof and subjects the sealing element and/or rotating shell to possible corrosion. Such prior art deficiencies have been approached with the teachings of U.S. Pat. No. 3,802,961 of Grass et al with considerable success.